1. Field of the Invention
The present invention relates to a color image detecting device for detecting color images and converting the same into electrical signals or the like. More particularly, it relates to improvements in a color image detecting device such as a CCD image pick-up device or the like, which comprises a color filter disposed in front of a photodetector having a surface comprising a number of regularly divided and arranged image element areas.
2. Description of the Prior Art
Reference is first made to FIG. 1 of the accompanying drawings to describe a conventional color image detecting device. In the ensuing description, for convenience of illustration, an example is presented in which a one-dimensional photodetector having image element areas arranged in one direction is used as the photodetector. It will be understood, however, that a two-dimensionally arranged photodetector or image pick-up tube can also be used according to the principles set forth herein.
In FIG. 1, an imaging optical system 3 serves to keep an object and the detecting surface of a detector 5 in a conjugate relation, and a point 1 of the object on the optical axis and a point 1' of the object off the optical axis are respectively imaged at different points 2 and 2' on the photodetecting surface. As shown in FIG. 2A of the accompanying drawings, just in front of the photodetector 5 having divided image element areas 50, 51, 52, . . . regularly arranged, there is disposed a color filter 7 having a number of filter elements 70, 71, 72, . . . arranged on a transparent substrate 6. Between the color filter 7 and the detecting surface of the photodetector 5, an adhesive layer 8 having a thickness d and a refractive index n is interposed to secure the two surfaces against relative movement therebetween.
In FIG. 2A, the light beam 41 from the point 1 (FIG. 1) of the object on the optical axis passes, for example, through a green color filter element 71 and forms a point image 2 on the image element area 51 of the photodetector 5 on the optical axis, and is converted into an electrical signal. The filter elements 70, 71, 72, . . . of the color filter 7 such as red, green, blue, etc. which have different transmitting characteristics and the image element areas 50, 51, 52, . . . of the photodetector are arranged so as to be opposed to each other at an equal pitch. In such a color image detecting device, correct color information is detected from the image of an object on or near the optical axis, as shown in FIG. 2A. However, as regards the points of an object off the optical axis, correct color information cannot be obtained, as shown in FIG. 2B. Such tendency becomes more pronounced as the object departs more from the optical axis. That is, the light beam 41' from the off-axial point 1' passes through a color filter element 91 and forms a point image 2' in the image element area 81 on the photodetector 5, but since the principal ray 21 is inclined with respect to the photodetecting surface, part 22 of the marginal rays of the light beam 41' does not pass through the color filter element 91 through which it should pass, but instead passes through an adjacent color filter element 92 to the image element area 81 and thus, the detected signal is an incorrect signal having two types of color information mixed together. Thus, in prior art conventional color image detecting devices, each of the elements of the color filter and each of the image element areas of the photodetector have been arranged at an equal pitch in opposed relationship with each other and this has led to the disadvantage that in the circumferential portion of the image, correct color information often cannot be detected.
As a solution to the above-noted disadvantage peculiar to conventional devices, it might occur to one to employ as the imaging optical system of FIG. 1 an optical system in which, even for the image off the optical axis, as shown in FIG. 3, the principal ray is incident perpendicularly to the imaging plane, namely, a so-called telecentric optical system whose exit pupil position lies at infinity. In a telecentric optical system, a diaphragm 4 must be placed on the forward focal plane of a lens system 31 which lies rearwardly of the diaphragm and therefore, it is difficult to reduce the full length of the imaging optical system while maintaining good performance. In addition, in order that the light beam leading this telecentric imaging optical system may cover the entire photodetecting surface, the diameter of its last lens surface 32 must be greater than the maximum dimension l of the photodetecting surface, and this undesirably leads to bulkiness of the imaging optical system itself. To make the imaging optical system compact without employing such a telecentric optical system, there are methods of bringing the color filter 7 and the photodetector into extremely intimate contact with each other. A first one of such methods is to make the thickness d of the adhesive layer 8 extremely thin. However, it is very difficult to render the thickness of the adhesive layer to several .mu.m or less, and it is also difficult to obtain a material compatible with the durability of the adhesive layer. A second method is to form a color filter layer directly on the photodetector. However, this method has the disadvantage that the manufacturing yields of the photodetector and color filter are considerably low and accordingly, the resultant device is very costly.